1. Field of the Invention
The invention relates to a telecommunication system comprising a supply circuit for a two-wire telecommunication line, the supply circuit comprising a voltage adjusting circuit section coupled between a first reference potential point and the first wire, and further comprising a current source coupled between the second wire and a second reference potential point. The voltage adjusting circuit section includes a transistor having a first electrode coupled to the first wire, a second main electrode coupled to the first reference potential point, and a control electrode coupled to one of the reference potential points and, by way of a capacitor, to the first wire.
2. Description of the Related Art
A telecommunication system of this type is widely used as a telephone exchange. Such a telephone exchange is connected to users by means of telecommunication lines comprising each a first and a second wire. During a user's conversation the associated telecommunication line is to be fed with DC current. This was previously effected by way of large coupled coils, but these coils have now been replaced by an electronic supply circuit. Such a supply circuit comprises a voltage adjusting circuit section coupled between a first reference potential point (usually ground) and the first wire of the line. The supply circuit further includes a current source coupled between the second wire of the line and a second reference potential point (usually-48 volts). The voltage adjusting circuit section comprises a transistor having a very high control input impedance, such as a MOSFET (for example of the N-channel type), although a double bipolar Darlington combination could be used as well. The first main electrode of the MOSFET, the source, is usually coupled to the first wire by way of a source resistor, whereas the second main electrode, the drain, is directly connected to ground. The control electrode, the gate, is coupled to the first wire by way of a capacitor and also usually coupled to ground by way of a gate resistor. The current source generally comprises a bipolar transistor (for example, of the n-p-n type), whose emitter and base are coupled to the-48 reference point by way of an emitter resistor and a zener diode respectively, and whose collector is connected to the second wire. The base is further coupled to ground by way of a series combination of a base resistor and a switch.
When a conversation is to take place, the switch is closed and a DC current starts flowing through the current source. This DC current may be adjusted to 30 mA DC by means of a specific selection of the components of the current source, and is supplied to the current source by the voltage adjusting circuit section. This takes place through both wires, which are mutually coupled and have a DC current impedance of about 600 Ohms. Since the capacitor is discharged at the moment the switch closes, the MOSFET is not conductive and in the first instance only a current much smaller than the adjusted 30 mADC current will flow from ground, through the gate resistor, the capacitor, the two wires, the bipolar transistor and the emitter resistor to the-48V reference point. Since the current source is passing much less than the adjusted 30 mADC current, the voltage across the current source is very low and so the voltage of the two wires (the line voltage) in the first instance is in the neighbourhood of the-48V reference potential. The capacitor is slowly charged by this small current until the voltage across the capacitor has increased sufficiently to render the MOSFET conductive. Because the main current path of the MOSFET then passes the adjusted 30 mADC current, the voltage across the current source will rise and the line voltage will increase to a value between the two reference potentials. The final value thus reached by the line voltage lies in the neighbourhood of the potential (ground) at the first reference potential point.
Such a prior-art supply circuit must meet strict requirements as to symmetry and demodulation, as is true of any telecommunication line supply circuit. The strict symmetry requirements are imposed in order to avoid low-frequency common mode disturbance signals from becoming audible. The strict demodulation requirements are imposed in order to avoid a high-frequency modulated common mode disturbance signal from becoming demodulated as a result of non-linear signal processing, and thus becoming audible in the case of insufficient symmetry. In the case of sufficient symmetry both requirements will be met. The described supply circuit will meet these requirements if the impedance of the series combination of the gate resistor and capacitor located between the ground first reference potential point and the first wire is made sufficiently large. In that case, however, there is the disadvantage that the time constant formed by such combination also increases likewise; so that the supply circuit will have a slower adjustment behaviour. This is undesired.